This invention relates to shutters for CCD imagers.
The invention is particularly applicable to CCD imagers having an image region, and a store region connected to the image region
Typically, the image region consists of a rectangular array of pixels, and the store region also consists of a rectangular array of pixels. Integration takes place in the image region for one field period. The image is then transferred rapidly from the image region to the store region, line by line, but during a substantially shorter period, known as the frame transfer period. This is done by applying clocked voltages to electrodes in the image region and in the store region. Then, while the next image is being built up in the image region, the image in the store region is clocked out line-by-line into an output register.
Typically, a shutter is used to blank off the optical signal to the image region of the CCD during the frame transfer period to prevent frame shift smear. Frame shift smear would result if the transfer to the store region took place with the image region uncovered, because any bright object detected in a pixel at the bottom of a column would appear in all pixels above it as the column was transferred to the store region. A mechanical rotary device with a blade that obscures the light path during frame transfer is a typical shutter.
The invention is especially concerned with CCD imagers, in which each integration period is in two portions, the second being of different sensitivity to the first. This may be due to signal clipping during the first part of the image, in order to extend the dynamic range of the imager. Such an imager is described and claimed in our concurrently filed and commonly owned U.S. patent application Ser. No. 12/726,050, the disclosure of which is incorporated herein by reference.
A conventional rotary shutter blade would now have a disadvantage if used to prevent frame shift smear because the effective integration period of the signal would no longer be constant over the area of the sensor.